The ascomycete Aspergillus nidulans exhibits two modes of development--asexual sporulation (conidiation) and sexual sporulation. The formation of conidia is inducible after a spore-originated colony has reached a certain age (competence). The aim of this project is to define the steps in the conidiation process by analysis of thermosensitive aconidial mutants. Temperature-shift experiments have shown that most aconidial mutants are blocked after induction and are termed maturation mutants. One exceptional mutant, however, has a thermosensitive function that is completed before induction. This mutant defines a function required for responsiveness to induction and thus required for the expression of all subsequent induction-dependent functions. Such a mutant is called a competence mutant. Many aconidial mutants are pleiotropic; some are blocked in the sexual cycle and others have an altered protein spectrum. Both modes of development are marked by the appearance of pathway-specific enzymes (laccases) which serve as event markers. Mutants defective in one or both of these enzymes have been isolated. The overall pattern of pleiotropy (of the aconidial and laccase-negative mutants) can be interpreted in terms of a coupling between the asexual and sexual pathways such that the early pathway (asexual) regulates the later (sexual) pathway. The phenotype of the aconidial competence mutant is affected by the allelic state of two other genes, the expressions of which are induction-independent. Particular allelic combinations of these three genes determine the relative vigor of sexual development. These studies will be extended to answer the following general question: For a given gene A what other genes must be expressed before gene A can be expressed? The complete answer to this question will give a description of the developmental pathway.